Peter Møller Juhl

A NUMERICAL AND EXPERIMENTAL INVESTIGATION OF THE PERFORMANCE OF SOUND INTENSITY PROBES AT HIGH FREQUENCIES

The influence of scattering and diffraction on the performance of sound intensity probes has been examined using a boundary element model of an axisymmetric two-microphone probe with the microphones in the usual face-to-face arrangement. On the basis of calculations for a variety of sound field conditions and probe geometries it is concluded that the optimum length of the spacer between the microphones is about one microphone diameter; with this geometry the effect of diffraction and the finite difference error almost counterbalance each other up to about an octave above the frequency limit determined by the finite difference approximation. This seems to be valid under virtually any sound field condition that could be of practical importance in sound power determination. The upper frequency limit corresponds to about 10 kHz for an intensity probe with 12-in. microphones, which means that it should be possible to cover most of the audible frequency range, say, from 50 Hz to 10 kHz, with a single probe conf...

Sound Exposure of Symphony Orchestra Musicians

BACKGROUND Assessment of sound exposure by noise dosimetry can be challenging especially when measuring the exposure of classical orchestra musicians where sound originate from many different instruments. A new measurement method of bilateral sound exposure of classical musicians was developed and used to characterize sound exposure of the left and right ear simultaneously in two different symphony orchestras. OBJECTIVES To measure binaural sound exposure of professional classical musicians and to identify possible exposure risk factors of specific musicians. METHODS Sound exposure was measured with microphones mounted on the musicians ears and recorded digitally. The recorded sound was analysed and the specific sound exposure of the left and the right ear was determined for the musicians. A total of 114 measurements covering 106 h were recorded in two symphony orchestras. RESULTS Sound exposure depends significantly on the specific instrument and the repertoire played by the exposed musician. Concerts, group rehearsals and individual practice were all significant contributors to the sound exposure. The highest L(Aeq) of 86 -98 dB was found among the brass players. High string players were exposed from 82 to 98 dBA and their left ear was exposed 4.6 dB more than the right ear. Percussionists were exposed to high sound peaks >115 dBC but less continuous sound exposure was observed in this group. Musicians were exposed up to L(Aeq8h) of 92 dB and a majority of musicians were exposed to sound levels exceeding L(Aeq8h) of 85 dB. CONCLUSIONS Binaural recording of the individual sound exposure showed that orchestra musicians could be exposed differently to the left and right ear and that they were primarily exposed from their own instruments. Specific repertoires as well as the specific instrument determine the level of exposure.

On the modeling of narrow gaps using the standard boundary element method

Numerical methods based on the Helmholtz integral equation are well suited for solving acoustic scattering and diffraction problems at relatively low frequencies. However, it is well known that the standard method becomes degenerate if the objects that disturb the sound field are very thin. This paper makes use of a standard axisymmetric Helmholtz integral equation formulation and its boundary element method (BEM) implementation to study the behavior of the method on two test cases: a thin rigid disk of variable thickness and two rigid cylinders separated by a gap of variable width. Both problems give rise to the same kind of degeneracy in the method, and modified formulations have been proposed to overcome this difficulty. However, such techniques are better suited for the so-called thin-body problem than for the reciprocal narrow-gap problem, and only the first is usually dealt with in the literature. A simple integration technique that can extend the range of thicknesses/widths tractable by the otherwise unmodified standard formulation is presented and tested. This technique is valid for both cases. The modeling of acoustic transducers like sound intensity probes and condenser microphones has motivated this work, although the proposed technique has a wider range of applications.

On the applicability of the spherical wave expansion with a single origin for near-field acoustical holography.

The spherical wave expansion with a single origin is sometimes used in connection with near-field acoustical holography to determine the sound field on the surface of a source. The radiated field is approximated by a truncated expansion, and the expansion coefficients are determined by matching the sound field model to the measured pressure close to the source. This problem is ill posed, and therefore regularization is required. The present paper investigates the consequence of using only the expansion truncation as regularization approach and compares it with results obtained when additional regularization (the truncated singular value decomposition) is introduced. Important differences between applying the method when using a microphone array surrounding the source completely and an array covering only a part of the source are described. Another relevant issue is the scaling of the wave functions. It is shown that it is important for the additional regularization to work properly that the wave functions are scaled in such a way that their magnitude on the measurement surface decreases with the order. Finally, the method is applied on nonspherical sources using a vibrating plate in both simulations and an experiment, and the performance is compared with the equivalent source method.

An axisymmetric boundary element formulation of sound wave propagation in fluids including viscous and thermal losses

The formulation presented in this paper is based on the boundary element method (BEM) and implements Kirchhoffs decomposition into viscous, thermal, and acoustic components, which can be treated independently everywhere in the domain except on the boundaries. The acoustic variables with losses are solved using extended boundary conditions that assume (i) negligible temperature fluctuations at the boundary and (ii) normal and tangential matching of the boundarys particle velocity. The proposed model does not require constructing a special mesh for the viscous and thermal boundary layers as is the case with the existing finite element method (FEM) implementations with losses. The suitability of this approach is demonstrated using an axisymmetrical BEM and two test cases where the numerical results are compared with analytical solutions.

A BEM approach to validate a model for predicting sound propagation over non-flat terrain

Abstract A two-dimensional boundary element model for sound propagation in a homogeneous atmosphere above non-flat terrain has been constructed. An infinite impedance plane is taken into account in the Greens function in the underlying integral equation, so that only the non-flat parts of the terrain need to be discretised in the boundary element model. This Greens function is undefined for points below the impedance plane, and therefore valleys and hollows are taken into account by coupling the exterior domain above the ground with one or several interior domains below the ground, as suggested in a recent paper [J. Sound Vibrat. 223 (1999) 355]. The resulting BEM model, which can handle arbitrary combinations of barriers and hollows, has been used for validating a ray model for various difficult configurations, including combinations of valleys and barriers.

User-operated speech in noise test: Implementation and comparison with a traditional test

Abstract Objective: The purpose of this study was to implement and evaluate a user-operated speech in noise test. Design: The test is based on the Danish speech material Dantale II, which consists of five words sentences (18). For each word presented the subject selected a response from ten alternative words. Two versions of the test were made: one with and one without the possibility that for each word presented the subject could answer “I do not know” (?-button). Using a listening test the two versions were evaluated against a traditional test, where the subjects orally repeated the words that were perceived. Study sample: Twenty-four normal-hearing subjects. Results: The speech intelligibility as a function of the signal-to-noise ratio can be described by logistic functions in the different user-operated tests and in the traditional test. The logistic parameters obtained from the user-operated test with the ?-button agree with the parameters obtained in a traditional test. The homogeneity of the speech material is uninfluenced when the material is used in a user-operated test. Conclusions: It is reasonable to use the Dantale II speech material for a user-operated speech in noise test, and the use of the ?-button is favourable.

A note on measurement of sound pressure with intensity probes

The effect of scattering and diffraction on measurement of sound pressure with “two-microphone” sound intensity probes is examined using an axisymmetric boundary element model of the probe. Whereas it has been shown a few years ago that the sound intensity estimated with a two-microphone probe is reliable up to 10 kHz when using 0.5 in. microphones in the usual face-to-face arrangement separated by a 12 mm spacer, the sound pressure measured with the same instrument will typically be underestimated at high frequencies. It is shown in this paper that the estimate of the sound pressure can be improved under a variety of realistic sound field conditions by applying a different weighting of the two pressure signals from the probe. The improved intensity probe can measure the sound pressure more accurately at high frequencies than an ordinary sound intensity probe or an ordinary sound level meter.

Practical modeling of acoustic losses in air due to heat conduction and viscosity

Accurate acoustics models of small devices with cavities and narrow slits and ducts should include the so‐called boundary layer attenuation caused by thermal conduction and viscosity. The purpose of this paper is to present and compare different methods for including these loss mechanisms in analytical and numerical models. A simple circular geometry with a narrow tube has been used as a reference and is investigated both through measurements and the different models. The simulation methods compared are: i) traditional analytical approaches such as lumped parameter modelling and transmission line modelling, ii) numerical methods implemented into commercial packages, such as the low reduced frequency models as proposed by W. M. Beltman and implemented in ACTRAN and the linearized Navier‐Stokes equations used in COMSOL Multiphysics, and iii) an implementation specifically made for this purpose using BEM and the full linearized model by M. Bruneau.

A comparison of SONAH and IBEM for near‐field acoustic holography

Among the popular techniques for acoustic source identification in complex environments are the Statistically Optimal Near Acoustic Holography (SONAH) and the Inverse Boundary Element Method (IBEM). These two methods are quite different regarding the underlying assumptions and the practical implementations: Whereas SONAH performs the back‐propagation of the sound field to a plane surface; the IBEM has no restrictions on the radiating geometry. On the other hand, IBEM requires the generation of a surface mesh and a time consuming solution process. The present paper compares the performance of the two methods for a number of simulated test cases and studies the influence on the performance of the models when changing selected parameters. Finally, the performance of the methods is compared in a simple measurement setup.